The development of surfactant-templated mesostructures represents a major advance in materials chemistry. Several attractive features, such as large surface areas, tunable pore sizes and volumes, and well-defined surface properties make mesostructured materials ideal for hosting molecules of various sizes, shapes, and functionalities. For example see Stein et al., Adv. Mater. 2000, 12:1403; and Sayari et al., Chem. Mater. 2001, 13:3151. Hexagonally ordered mesoporous silicate structures were discovered by Mobil Corp. (M41S materials like MCM-41) and by Kuroda et al. (FSM-16 materials). See, e.g., Kresge et al., Nature 1992, 359:710 and Yanagisawa et al., Bull. Chem. Soc. Jpn. 1990, 63:988 (1990). Structures of uniform pore sizes can now be formed throughout the mesopore size range, which encompasses 2-50 nm by IUPAC definition. See, Sing et al., Pure Appl. Chem. 1985, 57:603.
In the field of drug delivery, many site-selective deliveries, e.g., deliveries of highly toxic antitumor drugs, such as Taxol, require “zero release” before reaching the targeted cells or tissues. Unfortunately, the release of compounds from many drug delivery systems takes place immediately upon dispersion of the drug/carrier composites in water. For example, see Radin et al., J. Biomed. Mater. Res. 2001, 57:313; Aughenbaugh et al., J. Biomed. Mater. Res. 2001, 57:321; and Kortesuo et al., Int. J. Pharm. 2000, 200:223. The release mechanism of other systems, such as biodegradable polymer-based drug delivery systems, also relies on the hydrolysis-induced erosion of the carrier structure. See Uhrich et al., Chem. Rev. 1999, 99:3181; and Langer, Acc. Chem. Res. 1993, 26:537. Additionally, many polymeric based release systems require organic solvents for drug loading, which can trigger undesirable modifications of the structure or function of the encapsulated molecules, such as protein denaturation or aggregation. See Li and Kissel, Controlled Release 1993, 27:247.
The development of mesoporous silica-based carrier systems for controlled-release delivery of drugs, biocides, genes, or even proteins in vitro or in vivo is of keen interest. See Vallet-Regi et al., Chem. Mater. 2001, 13:308; Munoz et al., Chem. Mater. 2003, 15:500; Ramila et al., J. Sol.-Gel Sci. Technol. 2003, 26:1199; Diaz et al., J. Mol. Catal. B: Enzym. 1996, 2:115; Han et al., J. Am. Chem. Soc. 1999, 121:9897; Kisler et al., Microporous Mesoporous Mater. 2001, 44-45:769; Yiu et al., Microporous Mesoporous Mater. 2001, 44-45:763; and Takahashi et al., Microporous Mesoporous Mater. 2001, 44-45:755. Additionally, recent reports in the literature have demonstrated that polyamidoamine (PAMAM) dendrimers can serve as non-viral gene transfection reagents (Dennig and Duncan, Rev. Mol. Biotechnol. 2002, 90:339, and the references therein; Esfand and Tomalia, Drug Discovery Today 2001, 6:427). However, only those PAMAMs of high generations (G>5) have been shown to be efficient in gene transfection. The required procedures for the synthesis and purification of these high G PAMAMs are usually tedious and low-yielding. In contrast, the low G PAMAMs (G<3) are typically nontoxic and easily synthesized. Despite these benefits, the smaller molecular sizes and the limited surface charges of the low G PAMAMs prohibit efficient complexation with plasmid DNAs in solution due to the entropy penalty. Despite this current interest, there remains a need for novel carrier systems that can be used for controlled-release delivery.